1) Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a metal-made honeycomb body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor catalyst, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, the present invention is concerned with an exhaust gas cleaning device, of heat-resistant, planar and corrugated bands as constituent members of a metal-made honeycomb core body, at least the corrugated band defining minute pits in a surface thereof. Use of such a corrugated bands makes it possible to improve the joining strength of contact portions of both the bands, to say nothing of the retainability of the waveform of the corrugated band in the course of production of a honeycomb body and also the productivity of the honeycomb body.
2) Description of the Related Art
Exhaust gas cleaning devices of the above sort have conventionally been fabricated in the following manner. Namely, a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy form are superposed one over the other in a contiguous relation. They are rolled together into a multi-layered spiral form of many elements, each of which is composed of such planar and corrugated bands, and stacked one over another in layers, thereby forming a honeycomb shaped multi-layered composite body (hereinafter called the "honeycomb core body") axially defining a number of network-patterned gas flow passages (hereinafter called the "cells") for allowing exhaust gas to axially flow therethrough. The honeycomb core body is then inserted into a cylindrical metal casing which is open in both ends thereof.
There is however a recent move toward constructing an exhaust gas cleaning device without any separately-fabricated metal casing, in other words, with a metal-made honeycomb core body alone from the standpoint of the price competition with conventional cordierite ceramic carriers. The omission of such a metal casing can of course save the cost for the production of the metal casing and also the cost required for inserting a metal-made honeycomb core body in the metal casing and then uniting them together, namely, the so-called canning cost, whereby a substantial cost merit can be brought about accordingly.
In a metal-made exhaust gas cleaning device of this sort, the honeycomb core body, the metal casing as well as the planar band and the corrugated band, said bands constructing the honeycomb core body, are firmly united together by welding, brazing, mechanical joint means or the like so that they can withstand, for example, thermal expansion and thermal stress--which are developed by the high temperature of exhaust gas itself and exothermic reactions of exhaust gas, said reactions being induced by a cleaning catalyst (Pt, Pd, Rh or the like) supported on wall surfaces of the honeycomb body--or vibrations while an automotive vehicle on which the device is mounted is running.
A honeycomb core body, which is a principal component of an exhaust gas cleaning device of this sort as described above, is produced from heat-resistant steel sheets, more specifically, by forming a hoop of a desired width, namely, a planar band, for example, from a heat-resistant Fe-Cr20%-Al5% of 0.03-0.1 mm in thickness (or a heat-resistant stainless steel obtained by adding one or more rare earth metals to the former stainless steel to improve the oxidation resistance)--and corrugating such a planar band into a corrugated band and then superposing these bands one over the other in a contiguous relation.
In a process for the production of a honeycomb core body, for example, when a rolled honeycomb core body is produced by superposing a planar band and a corrugated band one over the other in a contiguous relation and then rolling the thus-superposed bands together into a multi-layered, spiral form, the most serious problem is that the waveform retainability (rigidity) of the corrugated band is overcome by rolling stress and many network-patterned gas flow passages (cells) formed after the completion of the rolling, said passages being adapted to permit passage of exhaust gas, become uneven. The cells therefore are in both shape and density, fail to efficiently produce a uniform honeycomb core body. There is thus no choice other than setting the rolling speed at a relatively low level and producing honeycomb core bodies under conditions which do not deform their waveform.
The above problem also arises in the case of stacked honeycomb core bodies and S-shaped honeycomb core bodies. A stacked honeycomb core body is produced by stacking many elements, each of which has been formed by superposing a planar band and a corrugated band one over the other in a contiguous relation, one over another in layers. On the other hand, an S-shaped honeycomb core body is produced by stacking such elements in layers such that planar bands define outermost surfaces respectively, and then by bending the thus-stacked elements about two fixed axes chosen on the upper and lower outermost surfaces respectively, such that each of the elements presents a substantially S-shape. There is another drawback in that, because external stress is applied in the course of production of the honeycomb bodies of the various types or upon their enclosure within casings, their waveforms are prone to deformation.
The low waveform retainability (rigidity) of the corrugated band during production of the honeycomb core body as described above has led to a significant disadvantage in the performance of the exhaust gas cleaning device. Non-uniformity in the shape and/or density of cells brings about serious problems such that the pressure loss of exhaust gas becomes substantial, resulting in lowered engine efficiency and also in failure to maintain uniform cleaning ability for exhaust gas.